A. The Field of the Invention
This invention relates to the field of devices used to provide roof and floor support in underground mines, especially coal mines. The invention is particularly useful for areas which require substantial roof support and which may be in danger of roof cave-in, including areas where roof support by wood cribbing has typically been used. The invention also helps prevent and minimize floor heave or buckling of a mine floor. The primary application of the invention is expected to be in longwall mining. The invention also has application in any underground mine where common wood cribbing or other methods are presently used to support the mine roof. The invention is particularly useful in preventing, delaying, and/or controlling both mine roof collapse and mine floor heave, reducing underground mine fire danger, and facilitating air and traffic flow within a mine.
B. The Background Art
Various roof support devices in the prior art have been designed and used to provide support to the mine roof. Deep mining results in removal of material from the interior of a mine, leaving unsupported voids of various sizes within the mine which may be in danger of collapsing. It is desirable to provide support to the mine roof to prevent, delay, or control collapse. Further, it is desirable for the mine roof support to be such that travel within the mine is not unduly restricted, that air flow within the mine remains adequate to support human life and to remove exhaust gases of various machinery in use in the mine, and that the danger of fire within the mine is not increased.
One possible method of mine roof support is to leave internal pillars of rock, coal, ore or other material to support the mine roof. The pillars are material which would normally be removed from the mine but for the need to support the mine roof. This method for supporting a mine roof is undesirable because the material which must be left in the mine to form the supportive pillars is usually coal or ore and represents substantial economic value to the mine owner. Further, no support is found for the mine roof between pillars and there may still be substantial danger of mine roof collapse.
Wooden beams or timbers have also been used in the past to provide mine roof support wooden beams have a serious safety disadvantage in their inability to yield and absorb load from the mine roof. Instead, they have a tendency to unexpectedly snap under load giving way to a mine roof collapse. Wooden beams are also subject to weakening over time due to decomposition, drying, cracking and splitting. The fire danger within a mine is increased with the presence of wooden beams. Wooden beams supported with wooden posts are also susceptible to the problems stated above.
Wooden posts have been tried as mine roof supports, with varying degrees of success. Single and multiple (ganged) wooden posts, of various diameters may be cut to fit between the mine roof and floor. The posts are held tight with wooden wedges and header boards at the top and/or bottom of the posts. These wooden posts are susceptible to the problems listed above and to catastrophic buckling.
The closest prior art to the present invention in current use in the mining industry today is wood cribbing. Traditional wood cribbing typically uses overlapping layers of two or more rectangular wood blocks stacked on each other in alternating fashion from the mine floor to the mine roof to form a roof support which is square in cross section and generally open in the center. The wood blocks may be of various sizes, including standard 8".times.8".times.48". The advantage of standard wood cribbing over other prior art mine roof supports is its combination of yield range, load support capacity, and stability. Wood cribbing will typically support a mine roof and yield to the compressive force of the mine roof over a wider range than many other alternative prior art mine roof supports. Traditional wood cribbing may continue to prove some roof support when it is crushed up to approximately 40% of its initial height. As it compresses, wood cribbing has been found to experience an increase in load carrying capacity of up to 400%. Both of these are desirable characteristics in a mine roof support. Typically, wood cribbing structure will buckle when crushed from 20% to 40% of its initial height, if the height to width ratio is less than two. This results in total loss of support characteristics and can lead to roof collapse and floor heave.
Wood cribbing has been more predictable than many other types of prior art mine roof supports, being less likely to collapse unexpectedly. Wood cribbing, however is subject to weakening over time due to decomposition, drying and cracking or splitting, it requires the use of expensive and sometimes difficult to obtain wood products, it must be assembled from multiple pieces of wood within the mine using costly human labor, and it will burn during a mine fire. Further, the shape and size of traditional wood cribbing cause some undesirable restriction to both traffic and air flow within the mine. Wood may be replaced by material such as autoclaved aerated concrete and steel mesh to achieve more long-term durability and fire resistance, but the other problems associated with traditional wood cribbing remain and the cost and difficulty of installation are increased.
Variations of traditional wood cribbing include donut and disk cribbing which comprise multiple donut or disk-shaped members stacked from mine floor to mine roof. Examples of this are Chlumecky (U.S. Pat. Nos. 4,565,469 and 4,497,597) and Deul (U.S. Pat. No. 5,143,484). The stacked donuts or disks are typically made of steel reinforced concrete although it would be possible to construct them from wood or other materials. Concrete donuts or disks do not deteriorate as quickly as wood and will not burn, but they are subject to cracking and crumbling because they are only yieldable over a limited load range. Further, the disks or donuts are heavy and require substantial human labor to install. Donut or disk cribbing has the advantage, however, of more readily facilitating traffic within the mine than traditional wood cribbing and providing less resistance to air flow.
An alternative method of cribbing uses telescoping pipe with a material within the pipe to provide yieldable resistance against pressure from the mine roof. An example of this is Thorn (U.S. Pat. No. 4,712,947). As pressure from the mine roof increases, a beam, pole or pipe telescopes within another pipe as the material within the pipe is compressed to absorb load. This type of mine roof support is costly to use in large numbers because of the various custom metal parts which must be employed. This type of mine roof support is also subject to unexpected and severe buckling and collapse when it is stressed beyond the limits of its load range. Further, if wood is employed as a component, there is no reduction in fire danger within the mine.